Two-step CO Research Articles

Tuning the product selectivity of single-atom catalysts for CO2 reduction beyond CO formation by orbital engineering.

Electrochemical CO2 reduction (CO2R) is one of the promising strategies for developing sustainable energy resources. Single-atom catalysts (SACs) have emerged as efficient catalysts for CO2R. However, the efficiency of SACs for the formation of reduction products beyond two-step CO formation is low due to the lower binding strength of the CO intermediate. In this study, we present an orbital engineering strategy based on density functional theory calculations and the fragment molecular orbital approach to tune product selectivity for the CO2R reaction on macrocycle based molecular catalysts (porphyrin and phthalocyanine) and extended SACs (graphene and covalent organic frameworks) with Fe, Co, and Ni dopants. The introduction of neutral axial ligands such as imidazole, pyridine, and trimethyl phosphine to the metal dopants enhances the binding affinity of the CO intermediate. The stability of the catalysts is investigated through the thermodynamic binding energy of the axial ligands and ab initio molecular dynamics simulations (AIMD). The grand canonical potential method is used to determine the reaction free energy values. Using a unified activity volcano plot based on the reaction free energy values, we investigated the catalytic activity and product selectivity at an applied potential of -0.8 V vs. SHE and a pH of 6.8. We found that with the imidazole and pyridine axial ligands, the selectivity of Fe-doped SACs towards the formation of the methanol product is improved. The activity volcano plot for these SACs shows a similar activity to that of the Cu (211) surface. The catalytic activity is found to be directly proportional to the sigma-donating ability of the axial ligands.

Dynamics of Two-Step CO Oxidation Light-Off on Pt/ $$\gamma$$ γ -Al $$_2$$ 2 O $$_3$$ 3 and Pd/ $$\gamma$$ γ -Al $$_2$$ 2 O $$_3$$ 3 in the Presence of C $$_3$$ 3 H $$_6$$ 6

Although the kinetics of CO and hydrocarbon (HC) oxidation reactions on three-way catalyst and diesel oxidation catalysts for automotive exhaust gas aftertreatment has been studied for a couple of decades, there are still some phenomena affecting the catalyst performance and pollutant conversion that need to be better understood and controlled. Two-step CO light-off is an undesired effect that significantly delays the process of reaching full CO conversion in the exhaust gas mixture including also hydrocarbons. In such a case CO light-off curve exhibits a plateau or shoulder where the CO conversion does not increase or even temporarily decreases with temperature, though there is enough O\(_2\) available for both CO and HC oxidation. The onset of hydrocarbon oxidation may inhibit the CO conversion due to blocking of active catalytic sites by adsorbed reaction intermediates. Furthermore, CO can be released as a by-product of the hydrocarbon oxidation. In this contribution we present the results of a systematic experimental study exploring these phenomena on Pt/\(\gamma\)-Al\(_2\)O\(_3\) and Pd/\(\gamma\)-Al\(_2\)O\(_3\) catalysts. Pt/\(\gamma\)-Al\(_2\)O\(_3\) showed larger decrease of CO conversion caused by accumulation of the surface intermediates. On the contrary, the production of CO by incomplete C\(_3\)H\(_6\) oxidation was higher on Pd/\(\gamma\)-Al\(_2\)O\(_3\). Overall extent of the two-step CO light-off effect was higher on Pt/\(\gamma\)-Al\(_2\)O\(_3\).

Cobalt-based ferrites as efficient redox materials for thermochemical two-step CO2-splitting: enhanced performance due to cation diffusion

The addition of CoO into CoFe2O4 triggers bulk cation diffusion and thereby leads to higher CO2 splitting reactivity.

Modeling of two-step CO oxidation light-off on Pt/γ-Al2O3 in the presence of C3H6 and NOx

Abatement of pollutants in automotive exhaust converters can be negatively affected by interactions of reaction intermediates with the catalyst surface. This paper focuses on a specific inhibition of CO oxidation on Pt/γ-Al2O3 catalyst by C3H6 oxidation intermediates, leading to a so-called two-step CO light-off behavior. Two reaction pathways contributing to a temporary decrease of CO conversion during a temperature-programmed co-oxidation of CO and C3H6 are identified: (i) accumulation of C3H6 oxidation intermediates on the catalyst surface that block active sites and progressively inhibit both CO oxidation and total C3H6 oxidation, and (ii) CO by-product formation during propylene oxidation. The latter pathway seems to play a minor role while the inhibition effect dominates. The phenomenon is most pronounced at lower (nearly stoichiometric) oxygen concentrations and in the presence of NO. This indicates that the initial formation of C3H6 oxidation intermediates is less sensitive to O2 and NO concentrations, while the total oxidation steps depend more strongly on the actual exhaust gas composition. A novel global reaction kinetic model is proposed that captures all the experimentally observed phenomena including transient accumulation of the reaction intermediates on the catalyst surface and the resulting impact on CO and C3H6 conversion. The developed model covers a wide range of operating conditions, from three-way catalyst (stoichiometric mixture) to diesel oxidation catalyst (lean mixture).

A 'Fluid' Nickel-Based Ferrite as an Efficient Redox Material for Thermochemical Two-Step CO <sub>2</sub> Splitting

The solar-driven, thermochemical two-step CO2 splitting reaction is essentially hindered by sintering of the redox material, and thermostable supports are often employed to alleviate this issue, although they usually play a limited role due to the harsh thermal condition. Herein, we demonstrate a distinct strategy, by engineering a “fluid” nickel-based ferrite material without using any support. It maintains a steady-state CO yield over 20 cycles, which is up to three times higher than that of the traditional NiFe2O4/ZrO2 material. Characterizations and first principles calculations suggest that its superiority can be attributed to the significantly enhanced flow of Fe cations, dramatically different from the NiFe2O4/ZrO2, where the oxygen ion undertakes the major transport. Such cation diffusion mode in our novel ferrite material provides a more accessible path to the bulk reaction, leading to a fast bulk reaction kinetics in the ferrite. Thus, we developed a new approach to overcome the sintering problem of ferrite materials.

Reactive stability of promising scalable doped ceria materials for thermochemical two-step CO2 dissociation

This work reports an improved and stable oxygen exchange capacity (OEC) of optimized doped ceria Ce1−xMxO2−δ (M = Zr, Hf, Nb) materials for two-step thermochemical CO2 splitting over 50 consecutive redox cycles (7 days).

Carbon monoxide release properties and molecular structures of phenylthiolatomanganese(I) carbonyl complexes of the type [(OC)4Mn(μ-S-aryl)]2.

Several phenylthiolatomanganese carbonyl complexes of the type [(OC)4Mn(μ-SR)]2 (R = Ph (), C6H4-4-CH3 (), C6H4-4-CF3 (), C6H4-4-F (), C6H4-4-Cl (), C6H4-4-OMe (), C6F5 (), and CH2C6H4-4-Cl ()) have been prepared via the reaction of Mn2(CO)10 with diaryldisulfane or via the reaction of [(OC)5MnBr] with arylthiols. These complexes lose two carbon monoxide molecules quite easily yielding tetranuclear [(OC)3Mn(μ3-SR)]4 (). Derivatives with fluoro-substituted aryl groups commonly form mixtures of dinuclear and tetranuclear which can quantitatively be converted to by heating of the corresponding reaction mixtures. A unique trinuclear structure is found for the mesityl derivative [(OC)4Mn(μ-SMes)]3 () which is maintained in solution as verified by IR and NMR spectroscopy. Traces of an already known dinuclear by-product of the type [(OC)3Mn(μ-SC6H3(-4-Me)-2-SC6H4-4-Me)]2 () have been structurally characterized. The suitability of [(OC)4Mn(μ-SPh)]2 () as a CO releasing molecule (CORM) for the administration of carbon monoxide has been studied. Two CO molecules are released upon dissolving in strongly Lewis basic solvents L, yielding [(OC)3Mn(L)(μ-SPh)]2, which liberates all the remaining CO molecules upon irradiation (photoCORM behavior).

Pd/Support Interface-Promoted Pd−Ce0.7Zr0.3O2−Al2O3 Automobile Three-Way Catalysts: Studying the Dynamic Oxygen Storage Capacity and CO, C3H8, and NO Conversion

The effect of the Pd−support interface on the dynamic oxygen storage capacity (DOSC) and three-way catalytic activities were investigated using Pd chromatographically distributed between Ce0.7Zr0.3O2 and Al2O3. CO−He pulse, H2-TPR, and XPS show that the Pd−(Ce, Zr) Ox interface promotes a higher degree of oxygen releasing than the Pd−(Al2O3) interface while maintaining the oxidized states of Pd. Pd promotion at the Pd−(Ce, Zr) Ox interface depends on the oxygen species of ceria−zirconia, and promotion takes place on surface oxygen and subsurface oxygen species. Dynamic CO−O2 transient measurement shows that DOSC is greatly promoted by the Pd−(Ce, Zr) Ox interface. The transient results indicate that the Pd−(Ce, Zr) Ox interface accelerates the oxygen spillover and back-spillover between Pd and the support oxide, but this phenomenon is not obvious for the Pd−(Al2O3) interface. Higher CO oxidation activity was obtained over a catalyst with a higher degree of Pd−(Ce, Zr) Ox interface in proportion to its DOSC performance. A two-step CO oxidation mechanism at the Pd−(Ce, Zr) Ox interface includes oxygen migration and its reaction with the adsorbed CO. Higher NO reduction and C3H8 oxidation rates were obtained with a catalyst with more Pd−(Al2O3) interface, showing negative correlation with the DOSC performance.

Activated Carbons Prepared from Extracted-Oil Palm Fibers for Nitric Oxide Reduction

In this paper, the preparation of activated carbons from extracted-oil palm fibers for the reduction of nitric oxide was studied. Preparation methods included two-step CO 2 activation, one-step CO 2 activation and chemical activation with KOH impregnation. The effects of the heating temperature and hold time on the properties of the chars and activated carbons were investigated. For chemical activation, the effects of KOH concentration and soak time were also studied. The activated carbon with the highest BET surface area was obtained by pyrolysis at 750 °C for 2 h and subsequent activation with CO 2 at 850 °C for 30 min for extracted-oil palm fibers impregnated with 0.10 M KOH for 24 h. Experimental results from the thermogravimetric study on the reduction of nitric oxide showed that activated carbons prepared from extracted-oil palm fibers could reduce NO x appreciably either by reaction with NO at high temperatures or by simultaneous reaction and adsorption for a mixture of NO and O 2 . These activated carbons can be potentially used for NO x reduction in flue-gas streams from high-temperature combustors or downstream of electrostatic precipitators and in some chemical plants operating at room temperature.

Reactive and selective redox system of Ni(II)-ferrite for a two-step CO and H 2 production cycle from carbon and water

A number of redox systems composed of carbon and bivalent metal-ferrites, MFe 2O 4(M = bivalent metal ions), were studied to find the most reactive and selective working materials for a thermochemical water-decomposition cycle combined with CO production from a carbon compound. Magnetite and Mg(II)-, Mn(II)-, Co(II)-, Ni(II)-, and Zn(II)-ferrites mixed with carbon powder have been screened for reactivity and selectivity in the CO-production step (first step) and subsequent water-decomposition step (second step). The Ni(II)-ferrite showed the most reactivity and selectivity for CO formation from carbon in the first step at temperatures above 700°C. The Ni(II)-ferrite could be completely reduced with carbon to the metallic phase of the NiFe alloy and Ni in the first step although only a small portion of magnetite was reduced to wustite under similar conditions. The ferrites reduced in the first step were oxidized with water vapor to generate H 2 in the second step. The highest conversion of H 2O to H 2 was obtained using the Ni(II)-ferrite. The total amount of evolved H 2 using Ni(II)-ferrite was 10 times larger than for the magnetite at 500°C. The processes could be repeated using the phase transition between Ni(II)-ferrite and NiFe alloy in the temperature range 700–800°C, with the highly efficient net reaction H 2O + C → H 2 + CO.

Facile and reversible CO insertion into the Ir–CH3bond of [Ir4(CH3)(CO)8(µ4-η33-Ph2PCCPh)(µ-PPh2)

Reaction of [Ir4H(CO)10(µ-PPh2)] with BuLi, Ph2PCCPh and then Mel gives [Ir4(CH3)(CO)8(µ4-η3-Ph2PCCPh)(µ-PPh2)], which undergoes a reversible two-step CO insertion under extremely mild conditions to yield lr4{(CH3C(O)}(CO)8–(µ4:η3-Ph2PCCPh)(µ-PPh2)] as the final product; the structures of both species have been established by X-ray diffraction studies.

Reaction of bis-µ-diethylphosphido-bis(tetracarbonylmetal)(M–M)(M = Cr or W) with tri-n-butylphosphine : kinetics and mechanism of a reaction involving seven-co-ordinate complexes

The complexes [(OC)4[graphic omitted](CO)4](M = Cr or W) undergo a two-step CO–PBu3 exchange, in decalin, giving [(Bu3P)(OC)3[graphic omitted](CO)4] and [(Bu3P)(OC)3[graphic omitted](CO)3(PBu3)]. All the substitutions for the forward and reverse reactions occur by a dissociative mechanism involving the reactive intermediate [L(OC)3[graphic omitted](CO)3](L = CO or PBu3). Values of the competition ratio k(CO)/k(PBu3)(up to 105) indicate that bimolecular attack of CO on this intermediate is largely favoured compared with that of PBu3. This unexpected sensitivity of the co-ordinatively unsaturated metal centre towards the nature of the incoming ligand is higher for chromium than for tungsten derivatives and increases from the first to the second substitution. Activation parameters corresponding to the rate constants and competition ratios, together with equilibrium data, for these reactions are reported.

Reaction of di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) with triphenyl-phosphine: a detailed kinetic and mechanistic study

Di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) undergoes a two-step CO substitution reaction with triphenyl-phosphine in decalin. The substitution does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. The unsubstituted complex undergoes direct attack by PPh3 either on the predominant anti-form or on the very reactive syn-form which is produced in the rate-determining anti–syn-isomerisation. The monosubstituted complex, which is also present mainly in the anti-form in equilibrium with a reactive syn-form, reacts with carbon monoxide through an SN2 mechanism, but a CO-dissociative mechanism is involved in its reaction with the bulkier PPh3. The bis(phosphine) complex so obtained is unable to undergo SN2 displacement even by carbon monoxide and must previously lose one molecule of phosphine. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediate by carbon monoxide and triphenylphosphine have been obtained. Equilibrium and activation parameters for these reactions are reported.